The humoral hypercalcemia of malignancy factor (HHMF) is a peptide which is secreted by certain malignant tumors and renders the affected patient hypercalcemic, an often lethal biochemical complication. Malignancy-associated hypercalcemia (MAHC) has been recognized as a common clinical problem ever since the first report of the case of a 69-year-old man with malignancy and hypercalcemia (H. Zondek et al., "Die Bedeutung der Calcium-bestimmung im Blute fur die Diagnose der Niereninsuffizietz," Z. Klin. Med. 99:128-29, 1923). Albright ("Case Records of the Massachusetts General Hospital (Case 27461)," N. Engl. J. Med. 225:789-91, 1941) described a patient with a renal carcinoma and a single skeletal metastasis. Because of the concomitant hypercalcemia and hypophosphatemia, Albright suggested that the tumor was secreting a systemic bone-resorbing factor that resembled parathyroid hormone in its actions. Subsequent studies confirmed that some tumor-induced hypercalcemia could be reversed by resection of isolatal malignant tumors (Plimpton et al., "Hypercalcemia in Malignant Disease Without Evidence of Bone Destruction," Am J. Med. 21:750-59, 1956; and Connors et al. , "The Etiology of Hypercalcemia Associated with Lung Carcinoma," J. Clin. Invest. 35:697-98, 1956). This enabled clinicians to identify the syndrome called "humoral hypercalcemia of malignancy" or "HHM." Patients with HHM develop hypercalcemia through elaboration of systemically acting, bone-resorbing factor or factors in tumor tissue remote from bone. These patients characteristically have squamous carcinomas or renal, bladder or ovarian carcinomas and have little or no evidence of skeletal disease (Andrew F. Stewart and Arthur E. Broadus, in Endocrinology and Metabolism, Felig et al. (eds.), 2d ed., McGraw-Hill, New York, 1987, pp. 1317-1453). The agent responsible for this paraneoplastic syndrome is not parathyroid hormone itself but is instead a novel peptide which is capable of interacting with parathyroid hormone receptors in some, but not all, target tissues.
The structure and sequence of the causative agent of HHM have heretofore eluded scientists. Experimental results (Stewart et al. [I], N. Engl. J. Med. 303:1377-83, 1980; Godsall et al., in Recent Progress in Hormone Research, Greep (ed.), 40, Academic Press, Orlando, Fla., 1986, pp. 705-50; Stewart et al. [II], "Identification of Adenylate Cyclase-Stimulating Activity and Cytochemical Glucose-6-Phosphate Dehydrogenase-Stimulating Activity in Extracts of Tumors from Patients with Humoral Hypercalcemia of Malignancy," Proc. Natl. Acad. Sci. USA 80:1454-58, 1983; Stewart et al. [III], "Frequency and Partial Characterization of Adenylate Cyclase-Stimulating Activity in Tumors Associated with Humoral Hypercalcemia of Malignancy," J. Bone Miner. Res. 1:267-76, 1986; Burtis et al., Endocrinology 118:1982-88, 1986; Insogna et al., "Biochemical and Histomorphometric Characterization of a Rat Model for Humoral Hypercalcemia of Malignancy," Endocrinology114:888-96, 1984; Goltzman et al., J. Clin. Endocrinol. Metab. 53:899-904, 1981; Broadus et al., "Messenger Ribonucleic Acid from Tumors Associated with Humoral Hypercalcemia of Malignancy Directs the Synthesis of a Secretory Parathyroid Hormone-Like Peptide," Endocrinology 117:1661, 1985; Strewlet et al., J. Clin. Invest. 71:769, 1983; Rodan et al., J. Clin. Invest. 72:1511-15, 1983; and Rabbani et al., Endocrinology 118:1200-10, 1986) have suggested that the responsible mediator may be a tumor-derived parathyroid hormone (PTH)-like adenylate cyclase-stimulating protein which acts upon PTH receptors in the skeleton and the kidney but which differs from native PTH with respect to size, immunoreactivity and encoding mRNA.
Stewart et al. II and Stewart et al. III describe the partial purification of a human HHM tumor-derived PTH-like adenylate cyclase-stimulating protein by virtue of its ability to stimulate a canine renal cortical, PTH-sensitive, adenylate cyclase assay and to stimulate a fetal bone resorption bioassay.
The PTH-Like Peptide is an adenylate cyclase-stimulating protein which acts through parathyroid hormone receptors but which is unrelated genomically to PTH. It was observed in patients with HHM when evaluating nephrogenous cyclic AMP (NcAMP) excretion that:
1. Patients with MAHC can be subdivided into two groups depending upon whether their NcAMP values are elevated or suppressed.
2. Those patients with elevated NcAMP have predominantly squamous and renal carcinomas, tumors commonly inked to the HHM syndrome. These patients have few or no bone metastases.
3. The groups of patients with suppressed NcAMP excretion have predominantly breast carcinoma or hematologic malignancies, and display evidence of widespread skeletal invasion by tumor.
4. Eighty-two percent (41 out of 50) of the patients evaluated were in the elevated NcAMP group, suggesting that HHM may be far more common than previously appreciated.
5. Normocalcemic patients with cancer have normal NcAMP excretion.
6. When compared to a group of patients with primary hyperparathyroidism, both HHM groups of patients have markedly elevated values for fasting or fractional calcium excretion, reduced values for circulating 1,25-dihydroxyvitamin D (1,25(OH).sub.2 D), and reduced or undetectable values for circulating and immunoreactive parathyroid hormone (PTH).
These results have been interpreted to indicate that the PTH-Like Peptide interacts with the same proximal tubular PTH receptor/adenylate cyclase complex as does PTH. Further, the PTH-Like Peptide is unlike PTH because: (1) the PTH-Like Peptide fails to stimulate the proximal tubular 1-hydroxylase responsible for 1.25(OH).sub.2 D synthesis; (2) the PTH-Like Peptide may be unable to stimulate distal tubular calcium reabsorption; and (3) the PTH-Like Peptide fails to interact, unlike PTH, with a series of region-specific PTH antisera.
Bone histology was investigated in patients with HHM and with primary hyperparathyroidism. Like patients with primary hyperparathyroidism, osteoclastic bone resorption is accelerated in HHM. In fact, the degree of bone resorption is strikingly increased beyond that encountered in primary hyperparathyroidism. Unlike patients with primary hyperparathyroidism, however, osteoblastic activity is markedly reduced in patients with HHM, indicating that bone cell activity is uncoupled in HHM, and also indicating that the mechanism of action of the PTH-Like Peptide on a given target organ may be similar to, but different from, the effects of PTH on the same target organ.
The PTH-Like Peptide is a PTH receptor agonist. This statement is supported by observations that:
1. Patients with HHM display elevated nephrogenous cyclic AMP excretion.
2. Extracts of tumors and tumor-conditioned medium from both human and animal HHM-derived tumors stimulate adenylate cyclase activity in renal cortical membranes and clonal osteoblast-like cells; and
3. Adenylate cyclase-stimulating activity is competitively inhibitable with synthetic PTH analogs, which indicates that these extracts or conditioned medium stimulate adenylate cyclase via PTH receptors.
Partial purification of PTH-Like Peptide through HPLC and SDS-PAGE slices found the peptide to be sensitive to oxidation but insensitive to extreme reducing conditions. These findings are similar to those of PTH (which contains methionine residues but no disulfide bonds).